A sticky polymer is one that tends to agglomerate at certain temperatures. It may be inherently sticky due to its chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers are also referred to as non-free flowing polymers because of their tendency to compact into aggregates of much larger size then the original particles and not flow out of the relatively small openings in the bottom of product discharge tanks or purge bins.
Three major process types are currently being used for the production of these sticky polymers, i.e., (1) The bulk monomer slurry process is quite efficient for contacting monomers with catalyst and obtaining high productivity. Some of the disadvantages associated with this process are the relatively high pressures used; and the very high concentration of dissolved monomer in the resin exiting the reactor. This type of process is characterized by a relatively small volume main reactor coupled to extensive monomer recovery/polymer flash and recovery facilities. (2) The diluent slurry process operates in a manner similar to the bulk monomer slurry process; however, the reactor tends to be larger and of lower pressure capability due to a lower monomer concentration requiring a larger reactor volume for the same rate of polymerization. The same disadvantages of the bulk slurry process are shared by the diluent slurry process. If the polymer is permitted to dissolve in the diluent, the solution viscosity increases drastically leading to reactor fouling. Relatively large diluent/monomer recovery systems must be maintained to economically recover same from purge bins and product recovery systems. (3) The solution process allows operation at higher reaction temperatures with improved heat removal. It also allows high polymerization rates for given reactor size due to the usually positive effect of reaction temperature on the activity of the polymerization catalyst. A major disadvantage of the solution process is the typically cumbersome recovery methods required for polymer and solvent. These methods require a significant amount of equipment and produce the final polymer in pellet form, which can be difficult to purge of residual monomer and solvent to environmentally safe and acceptable levels. One additional factor involved in the use of a solution process is the presence of a large volume of hot solvent, frequently well above the flash point, which contains a significant amount of dissolved polymer Spills of this type of material present significant hazards with regard to fire and personnel exposure. A further disadvantage of the solution process is that the rate and, to some extent, the operability of the system is dependent on the molecular weight and solubility of the particular product being produced. Products of higher density than desired (or designed for) will precipitate in cool spots in the lines and foul the reaction system. Products of higher molecular weight will increase solution viscosity to the point that the design capability of the circulation system will be exceeded and operations will become impractical. A major product deficiency of the solution processes is that they are inherently incapable of production of desirable high molecular weight or ultrahigh molecular weight grades of resin.
The sticky polymers, which are produced with varying degrees of success by the aforementioned processes, are for the most part elastomers such as ethylene/propylene rubbers and ethylene/propylene diene termonomer rubbers, high ethylene content propylene/ethylene block copolymers, poly(1-butene) (when produced under certain reaction conditions), very low density (low modulus) polyethylenes, e.g., ethylene/butene rubbers or hexene containing terpolymers, and ethylene/propylene/ethylidene norbornene terpolymers of low density.
While all of the problems raised by these liquid phase processes cannot be readily overcome, industry is constantly seeking to select, and improve on, the most efficient of the processes, particularly with the reduction of agglomeration in mind.